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LM4670 Datasheet(Fiches technique) 15 Page - National Semiconductor (TI)

[Old version datasheet] Texas Instruments acquired National semiconductor. Click here to check the latest version.
Numéro de pièce LM4670
Description  Filterless High Efficiency 3W Switching Audio Amplifier
Télécharger  18 Pages
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Fabricant  NSC [National Semiconductor (TI)]
Site Internet  http://www.national.com
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Application Information (Continued)
REFERENCE DESIGN BOARD SCHEMATIC
In addition to the minimal parts required for the application
circuit, a measurement filter is provided on the evaluation
circuit board so that conventional audio measurements can
be conveniently made without additional equipment. This is a
balanced input, grounded differential output low pass filter
with a 3dB frequency of approximately 35kHz and an on
board termination resistor of 300
Ω (see schematic). Note
that the capacitive load elements are returned to ground.
This is not optimal for common mode rejection purposes, but
due to the independent pulse format at each output there is
a significant amount of high frequency common mode com-
ponent on the outputs. The grounded capacitive filter ele-
ments attenuate this component at the board to reduce the
high frequency CMRR requirement placed on the analysis
instruments.
Even with the grounded filter the audio signal is still differ-
ential, necessitating a differential input on any analysis in-
strument connected to it. Most lab instruments that feature
BNC connectors on their inputs are NOT differential re-
sponding because the ring of the BNC is usually grounded.
The commonly used Audio Precision analyzer is differential,
but its ability to accurately reject fast pulses of 350ns width is
questionable necessitating the on board measurement filter.
When in doubt or when the signal needs to be single-ended,
use an audio signal transformer to convert the differential
output to a single ended output. Depending on the audio
transformer’s characteristics, there may be some attenua-
tion of the audio signal which needs to be taken into account
for correct measurement of performance.
Measurements made at the output of the measurement filter
suffer attenuation relative to the primary, unfiltered outputs
even at audio frequencies. This is due to the resistance of
the inductors interacting with the termination resistor (300
Ω)
and is typically about -0.25dB (3%). In other words, the
voltage levels (and corresponding power levels) indicated
through the measurement filter are slightly lower than those
that actually occur at the load placed on the unfiltered out-
puts. This small loss in the filter for measurement gives a
lower output power reading than what is really occurring on
the unfiltered outputs and its load.
20089909
FIGURE 8.
www.national.com
15




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Numéro de composants électroniques

Numéro de pièceDescription des composantsHtml ViewFabricant
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